The present invention relates to methods and apparatus for coding and decoding moving pictures.
Moving pictures are transmitted in digital form in a variety of systems, which will soon include digital television broadcasting systems. To reduce the quantity of transmitted data, digitized moving pictures must be coded at the transmitter and decoded at the receiver. The MPEG-2 coding system standardized by the Moving Picture Experts Group is widely used. In this system, a moving picture is coded predictively by a series of steps involving motion estimation, motion compensation, a discrete cosine transform (DCT), quantization, and variable-length coding. The coded data are decoded by a reverse process involving variable-length decoding, inverse quantization or dequantization, an inverse discrete cosine transform (IDCT), and motion compensation.
A general problem in this and other similar coding systems is their high computational cost. A significant part of the cost occurs in the IDCT, which involves many multiplication operations, the efficient performance of which requires extra hardware and software. Another cost occurs because of the large amount of semiconductor memory required for storing the reference pictures used in motion compensation. The many accesses required to this memory are yet another computational cost. These costs are particularly great when the transmitted picture is a high-definition television (HDTV) picture.
It would obviously be desirable to reduce these costs, particularly in broadcast receivers, the high price of which hinders the acceptance of digital broadcasting.
It would also be desirable to enable HDTV transmissions to be received by receivers equipped with standard-definition display screens, which are less expensive than high-definition display screens. This requires a process of resolution conversion.
An object of the present invention is to reduce memory requirements in a moving-picture decoder.
Another object of the invention is to reduce processing requirements in a moving-picture decoder.
Another object is to reduce memory requirements in a moving-picture coder.
Another object is to reduce processing requirements in a moving-picture coder.
A further object is to reduce the resolution of a decoded moving picture during the decoding process.
According to a first aspect of the invention, a coded moving-picture signal is decoded by the steps of:
performing an IDCT with resolution conversion, thereby generating a first picture signal with reduced resolution;
performing motion compensation, using a reduced-resolution reference picture signal, to generate a second picture signal with reduced resolution;
adding the first and second picture signals to generate an output picture signal; and
storing the output picture signal for use as the reference picture signal.
According to a second aspect of the invention, an input signal representing a moving picture is coded by the steps of:
comparing the input signal with a reference picture signal having reduced resolution, selecting predictive coding or intra-frame coding, and generating motion vectors for predictive coding;
applying the motion vectors to the reference picture signal, when predictive coding is selected, and performing resolution conversion, thereby obtaining a first picture signal having full resolution and a second picture having reduced resolution;
generating a third picture signal by subtracting the first picture signal from the input signal, when predictive coding is selected;
performing a DCT on the third picture signal, obtaining a transformed picture signal;
performing an IDCT with resolution conversion on the transformed picture signal, obtaining a fourth picture signal having reduced resolution;
adding the second and fourth picture signals to obtain the reference picture signal; and
storing the reference picture signal in a memory.
The first and second aspects of the invention may be combined to form a moving-picture coding and decoding system.
According to a third aspect of the invention, a predictively coded moving-picture signal is decoded by the steps of:
decoding the coded moving-picture signal with reference to a reference picture signal, obtaining an output picture signal;
compressively coding the output picture signal;
storing the compressed picture signal in a memory; and
expanding the compressed picture signal to obtain the reference picture signal.
The first and third aspects of the invention may be combined.
According to a fourth aspect of the invention, a one-dimensional IDCT is performed in a first mode with full resolution, and a second mode with reduced resolution, by the steps of:
obtaining first intermediate results by performing a first matrix operation on a first input signal;
obtaining second intermediate results by performing a second matrix operation on the first input signal in the first mode, and the first matrix operation on a second input signal in the second mode;
combining the first and second intermediate results by a butterfly calculation;
selecting the combined results of the butterfly calculation for output in the first mode; and
selecting the first and second intermediate results for output in the second mode.
The fourth aspect of the invention is applicable in the first and second aspects of the invention.
The invention also provides coding and decoding apparatus employing the invented methods.